A gas tank is not the sort of thing most people put on your “Must see” list. In the Prague suburbs, there is one of the world’s last surviving gas tanks with a preserved outer casing.
I held on as the tram #1 creaked its way east of Prague centre. I am surrounded by people who ignore me as just another commuter. Little do they know I am on my way to see a gas tank with great engineering heritage.
In July 1932, the Czechoslovaks built this 270-tonne spherical gas tank in the eastern Prague suburb of Liben. This industrial district was growing, needing large gas reserves close by. It was the first of its kind in Czechoslovakia and a technical marvel in Europe.  Spherical gas tanks were common because this shape makes it easier to store gas at higher pressure. This way, they could store larger concentrations of gas in a smaller volume. The construction was imposing and became a symbol of the industrialisation of the neighbourhood. 
The tram continues its way across the decrepit Libensky Most (Liben bridge). Summer has already turned to autumn here. Along with the cooler temperature, the sky seems grayer even if the cloud cover is not uniform. Our tram gets shunted and heads south, climbing part of the Vitkov hill.
The twenty metre diameter sphere has eight sheet-metal legs anchored in a concrete base for solidity. It was in service for about 10 years before German forces damaged at the end of World War II. A German plane shot at it, and a grenade exploded inside it. To the locals’ relief, it was empty at the time. While the authorities repaired the damage, you can still see machine-gun bullet holes today.  
We climb the tram bridge that arches over dual carriageways and train tracks. I crane my neck as I look for the gas tank. I catch glimpses of it between the treetops. It is a dull gray and is almost camouflaged against the sky until I blink and focus on it.
Once damaged, the tank could no longer be used for gas so instead the Aeronautical Research and Test Institute took it over. They realised it could be put to good use as a wind tunnel instead. This became the largest such wind tunnel in Czechoslovakia. They run tests on scale models of new planes to see if it’s worth building full prototypes. This facility now runs tests on simulated air speeds up to Mach 3.5 – 3.5 times the speed of sound. That’s 4,321 kilometres (2,685 miles) per hour to you and me. 
I get off the tram at the next stop, ignoring looks from people who wonder what I’m doing in the middle of nowhere. I catch the next tram in the opposite direction. We ride over the bridge again, keeping a sharp eye open for the gray bubble on the Prague landscape.
Today the sphere is not as visible as it once was. Partially this is because of rapid gentrification of the districts. Partially it is because of a number of poplar trees the city planted in the 1960s as camouflage.
The best way to see the tank is to wait for the annual European Heritage Days. I’ve missed it this year but have bookmarked it for 2018. It’s the only time the place is open to the public. At other times, you can do what I did and catch tram 1 from the Palmovka metro stop, heading south. These trams cross a bridge that provides a panoramic view across the valley and Prague; and the sphere itself. 
Have you visited any sites of industrial engineering? Leave a comment and let us know!